Abstract: A two-stage method of producing N.sub.2 O.sub.5 in nitric acid from N.sub.2 O.sub.4 in nitric acid consists of a first production stage in which the anodic oxidation of N.sub.2 O.sub.4 in nitric acid and cathodic reduction of nitric acid are separated by a non-ionic or anionic ion exchange membrane, and a second production phase in which the product of the first stage anodic reaction is subjected to further anodic oxidation, the anodic and cathodic reactions being separated by a cationic ion exchange membrane. The use of a cationic membrane in the second stage promotes, through leakage of N.sub.2 O.sub.4 to the catholyte, an increase in N.sub.2 O.sub.5 concentration and decrease in N.sub.2 O.sub.4 concentration within the anolyte acid while avoiding a significant loss of current efficiency. The two stages may be connected in series and operated continuously to produce a nitric acid solution containing typically 32 wt. % N.sub.2 O.sub.5 and less than 2 wt. % N.sub.2 O.sub.4.